In electrostatographic printers, printing parameters such as primary charger setpoint, exposure setpoint, toner concentration, and development bias must be periodically adjusted in order to maintain consistent color characteristics in the images being printed. Printer process control strategies typically involve measuring the reflective optical density of a toner image on an exposed and developed area on a printed sheet (called a “test patch”). Optical density has the advantage, compared to transmittance or reflectance measures, of matching more closely to human visual perception. A further advantage of relying on an optical density measurement to maintain consistent color characteristics in the printed images is that density is approximately proportional to the thickness of the marking material layer over a substantial range. The optical sensors used to make such reflection density measurements are known as densitometers. Such densitometers include an array of photo-transistors covered with a mask of color filters, and a light source that provides white light at a constant intensity. In operation, the photo-transistor array generates separate pulse frequency signals indicative of the density of selected light wavelengths (which typically correspond to red, blue, and green) as it scans the test patches of color.
An “in-line” densitometer refers to a densitometer that is mounted on the printer itself, and which measures the reflective density of test patches on printed sheets moving through a paper path in the printer. Density measurements are transmitted to the digital color controller of the printer as the densitometer scans the moving sequence of test patches (which are typically a series of cyan, magenta, yellow, gray and black rectangles) on the printed test sheets. From the input provided by the in-line densitometer, the digital color controller of the printer can determine whatever adjustments might be necessary to the color process control parameters to maintain consistent color characteristics in the printed images.
As indicated in FIG. 1, one location where an in-line densitometer 1 may advantageously be mounted on an electrostatographic printer is in the top plate of the paper transport section 3 immediately downstream of the fuser roller 5. This transport section 3 is sometimes referred to as the “fuser extension,” and includes a horizontally oriented bottom plate 7 to support the printed sheets 9 during transport. Often, due to the possibility of paper jams, this paper transport section 3 includes a top plate 11 having hinges 13 that allows it to be opened and then closed after the paper jam is cleared. Paper 9 that passes through this transport section 3 is propelled by pinch rollers 15a, b in the direction “A” but otherwise is free within the confines of the top and bottom plates 7, 11.
In order for the densitometer to provide consistent and accurate image density data to the digital color controller of the printer, it is necessary to maintain a constant vertical distance X between the array of phototransistors and sheets 9 printed with the test patches. The criticality of maintaining such a constant distance is illustrated in the graph of FIG. 2, where the horizontal and vertical axes represent phototransistor spacing (in millimeters) to the test sheets and the difference (or minimum delta) in the pulse frequency output of the densitometer indicative of a measured color density, respectively. As is evident from this graph, a variation of 0.50 millimeters from an optimal distance of about 2.3 millimeters will cause a minimum error of about 100 pulses per second to occur in images having mid to high saturation. Such a 0.50 millimeter variation in X may occur, for example, by the fluttering of the leading edge of the paper 9 as it is propelled by the pinch rollers 13a, b. The resulting minimum error of about 100 pulses per second in turn corresponds to a density measurement error of between about 1% and 5%, which is sufficient to result in perceptible variations in the color characteristics of a single image being printed multiple times.
One prior art solution to this problem is the provision of an optical system that compensates for variations in X. However, such systems require the use of a custom-made arrangement of precision lenses and hence are relatively complicated and expensive. Moreover, inaccurate measurements can still occur in situations where the vertical distance X varies beyond the capacity of the optical system to compensate. Another prior art solution seeks to maintain the vertical distance X by providing a precision-made top latch and hinge for the accurate re-positioning of the top plate relative to the bottom plate of the paper transport section, in combination with a spring-loaded mounting between the housing of the densitometer and the top plate to hold the paper in sliding engagement against the supporting, bottom plate. However, even when such mechanical components are provided, the applicant has observed that the vertical orientation of the typically metallic top plate in the paper transport section can vary a millimeter or more due to thermal differential expansion as a result of the combined variable heat output from the fuser roller located immediately upstream and the opening/closing action of the cover.
Clearly, there is a need for an in-line densitometer mountable in the fuser extension transport section of an electrostatographic printer that provides reliable color density measurements without the need for lenses or precision mechanical mounting components and overcomes all of the aforementioned disadvantages associated with prior art designs.